International audienceThe aim of inverting seismic waveforms is to obtain the “best” earth model. The best model is defined as the one producing seismograms that best match (usually under a least‐squares criterion) those recorded. Our approach is nonlinear in the sense that we synthesize seismograms without using any linearization of the elastic wave equation. Since we use rather complete data sets without any spatial aliasing, we do not have the problem of secondary minima (Tarantola, 1986). Nevertheless, our gradient methods fail to converge if the starting earth model is far from the true earth (Mora, 1987; Kolb et al., 1986; Pica et al., 1989)
Surfactant related chemical flooding has great potential for improving oil recovery in carbonate reservoirs, as surfactants are able to effectively lower the oil/water interfacial tension (IFT) and alter wettability toward water-wet. The loss of surfactant during the propagation process in the reservoir is one of the major concerns in determining the feasibility of a chemical flooding project. Many efforts have been made to reduce the surfactant adsorption in various scenarios to make the project economically successful. The addition of polymer in the chemical formulation is commonly used in the applications, which can not only be used to control the water/oil mobility ratio but also affect the surfactant adsorption in different ways.This paper presents the results of a laboratory study on the effect of polymer on reducing the surfactant adsorption onto carbonate reservoir cores. An amphoteric surfactant and a sulfonated polyacrylamide were used in three series of dynamic adsorption tests conducted at reservoir conditions, including surfactant only, polymer followed by surfactant, and mixture of surfactant and polymer (SP). The experimental results showed that when surfactant alone was injected, the surfactant adsorption on the carbonate reservoir cores ranged from 0.125 to 0.203 mg/g-rock, with an average value of 0.163 mg/g-rock. When the surfactant was injected following the injection of a pre polymer slug, the surfactant adsorption reduced to 0.0739 to 0.0848 mg/g-rock, with an average reduction of 51.3%. If SP mixture was injected, the surfactant adsorption ranged from 0.0794 to 0.0872 mg/g-rock, with an average reduction of 48.9%. It indicates that the adsorptions of surfactant and polymer take place competitively on the sites of the rock surfaces. This laboratory study helps understand the synergetic effect of surfactant and polymer in SP formulations and design the injection schemes of chemical flooding processes.
Full‐waveform inversion of seismic reflection data is highly nonlinear because of the irregular form of the function measuring the misfit between the observed and the synthetic data. Since the nonlinearity results mainly from the parameters describing seismic velocities, an alternative to the full nonlinear inversion is to have an inversion method which remains nonlinear with respect to velocities but linear with respect to impedance contrasts. The traditional approach is to decouple the nonlinear and linear parts by first estimating the background velocity from traveltimes, using either traveltime inversion or velocity analysis, and then estimating impedance contrasts from waveforms, using either waveform inversion or conventional migration. A more sophisticated strategy is to obtain both the subsurface background velocities and impedance contrasts simultaneously by using a single least‐squares norm waveform‐fit criterion. The background velocity that adequately represents the gross features of the medium is parameterized using a sparse grid, whereas the impedance contrasts use a dense grid. For each updated velocity model, the impedance contrasts are computed using a linearized inversion algorithm. For a 1-D velocity background, it is very efficient to perform inversion in the f-k domain by using the WKBJ and Born approximations. The method performs well both with synthetic and field data.
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